Radiation-curable, magnetic coating material and magnetic recording medium using the same

ABSTRACT

This invention concerns a radiation-curable, magnetic coating material and a magnetic recording medium made with the coating, which is characterized in that a water-soluble or water-dispersed resin having at least one radiation-curable, unsaturated double is contained as a vehicle. The vehicle may also contain a water-soluble or water-dispersible resin, or mixtures thereof free from such an unsaturated double bond. The invention provides a magnetic recording medium with improved durability, softness, flexibility, and heat resistance.

This application is a continuation of application Ser. No. 781,200,filed Sept. 26, 1985, now abandoned which is a continuation ofapplication Ser. No. 480,958, filed Feb. 28, 1983, now abandoned.

This invention relates to a magnetic coating material curable uponexposure to radiation and also to a magnetic recording medium obtainedby using the coating material.

Presently, varied forms of magnetic recording media, including cassettetapes, open reel tapes, magnetic cards, and magnetic disks, are made bydispersing or dissolving powdered iron oxide or suchlike magneticsubstance and a binder in an organic solvent, adding to the solution aplasticizer, dispersant, antistatic agent, and/or other additives whichmay be desired according to the intended use, mixing and kneading themixture, and then applying the resulting coating material on a base filmof polyester, polyvinyl chloride, polyacetate, or paper.

While various thermoplastic resins are still in use, it is a morecustomary practice, in order to obtain greater resistance to wear andheat and improve other physical properties of the product, to use athermosetting resin as the binder and bake (cure) the coating in an ovenfor many hours (e.g., at 70° C. for 2-20 hours). Besides the complexityof the process, the practice presents problems of blocking of thecoating, deformation of the tape due to tightening on winding, loss ofsmoothness on the magnetic coating surface, and other factors adverselyaffecting the electrical properties of the product.

Furthermore, the magnetic coating material of the type dispersed ordissolved in an organic solvent requires such a large volume of thesolvent that the process poses not only fire hazards but alsoenvironmental hygiene problems. For these reasons, the employment ofaqueous resin type magnetic coating material instead has been proposedin recent years.

The aqueous resin type precludes the fire and health hazards associatedwith the use of the organic solvent type. The proposed method frequentlyuses a thermoplastic resin but more often prefers a thermosetting oneand, as is the case with the organic solvent tape, the coating materialapplied on the substrate is baked (cured) over a long period of time inan oven so that the coating film will be more resistant to wear and heatand possess better physical properties. However, the method again offersproblems, in addition to the complexity of the process, such as blockingof the coating, tape deformation due to tightening on winding, loss ofsmoothness on the coating surface, and other deleterious effects on theelectrical properties of the product.

The present invention has been arrived at after intensive investigationsmade in search of an aqueous magnetic coating material adapted for usein making a magnetic recording medium which presents no such problems ofthe prior art but is excellent, especially in wear resistance andelectrical properties. The magnetic coating material according to thisinvention will harden at a low temperature within a short time uponirradiation with electron beams or other radioactive rays. The material,which cures continuously as it is applied and dried, makes for improvedproductivity and causes no blocking due to takeup before curing nor anyexcessive tightening on winding. Presumably, irradiation with electronbeams or the like causes a resin in the material which has at least oneunsaturated double bond to produce radicals, which in turn initiatepolymerization or cross-linking to form a three-dimensional networkstructure with good resistance to wear and heat and with other desirablephysical properties. Thus, the present invention provides an excellentradiation-curable, magnetic coating material, hazardless and highlydesirable from the viewpoint of environmental hygiene and which can beused in manufacturing a magnetic recording medium excellent indimensional stability, wear resistance, surface smoothness, and otherproperties, within a short time in a simplified process.

In brief, the invention provides a radiation-curable, magnetic recordingmedium which comprises as essential components an aqueous vehiclecontaining of a water-soluble resin, a water-dispersible resin, ormixtures thereof each having at least one unsaturated double bondcurable upon exposure to radiation, and a magnetic powder. Further, theinvention provides a radiation-curable, magnetic coating material whichcomprises as essential components an aqueous vehicle prepared by mixinga water-soluble resin and/or a water-dispersible resin, each having atleast one unsaturated double bond curable upon exposure to radiation,with a water-soluble resin and/or a water-dispersible resin, neither ofwhich has such an unsaturated double bond, and a magnetic powder.

Although magnetic recording media consisting of a base coated with amagnetic coating material which comprises an aqueous vehicle of athermoplastic resin, and a magnetic powder are known in the art, theresulting coats are simply dried by heating and therefore are inferiorin resistance to wear and heat, etc.

For the purposes of the invention the term "radiation-curable,unsaturated double bond" is used to mean a radical-polymerizable doublebond, e.g., an acrylic double bond such as of methacrylic acid or itsester, allylic double bond such as of diallyl phthalate, vinylether-vinyl ester type double bond, or unsaturated double bond of maleicacid or its derivative.

In short, the binder to be used in this invention is a resin with suchan unsaturated double bond or bonds in its main chain, side chain, or atboth ends of the molecular structure. Also, it is well-known thatthermoplastic resins readily undergo a cross-linking reaction uponirradiation and that the reaction is accelerated by the presence of aresin having at least one unsaturated double bond. Thus, even athermoplastic resin having no unsaturated double bond is caused, uponexposure to radiation, to take part in a cross-linking, curing reactionwith the resin having at least one unsaturated double bond to form athree-dimensional network structure. Even when the thermoplastic resindoes not form a cross-linked structure due to irradiation by a lowdosage of radiation, it does form a mutually intruded structure with thethree-dimensional network structure produced by the resin having atleast one unsaturated double bond. It thus acts as a polymeric filler orplasticizer, which imparts proper hardness or softness and good physicalproperties to the magnetic recording medium.

Experiments indicated the effects of molecular weight upon theradiation-curable resins. Where high-molecular-weight resins were used,magnetic recording media excellent in physical properties, especially inthe durability and softness required of magnetic tapes and the like,were obtained. The cross-linking, curing reaction further improved thethermal resistance. On the other hand, the higher the molecular weightthe less the functional group density became, and this effects atendency toward lower radiation curability. Therefore, a relatively highdosage of radiation is necessary. Where a resin with a low molecularweight of less than 2000 was employed, a resinous composition with goodradiation curability was obtained. However, the coating film made of thecomposition containing the low-molecular-weight resin was not adequatelysoft, and an increase in the percentage of the magnetic powder in thecomposition gave a brittle magnetic coating, which was notwear-resistant enough for use in making a magnetic tape. For therealization of the present invention, further investigations were madeto find ways of balancing the radiation curability with physicalproperties, especially the durability, softness, and heat resistance,desired of the coating film. As a result, it has now been found that acoating film having desirable physical properties is obtained withincreased functional group density, improved radiation curability, and adecreased dosage of radiation required, by using an aqueous bindercomposed of a water-soluble resin and/or a water-dispersible resin, eachhaving a molecular weight of at least than 5000, preferably at least8000, and curable upon exposure to radiation, together with awater-soluble resin and/or a water-dispersible resin, each having amolecular weight in the range of 200-5000, preferably in the range of400-3000, and curable upon exposure to radiation.

The resins having at least one unsaturated double bond and which may beemployed in the present invention include the compounds classifiableinto four categories I to IV to be given later. Of those compounds, somecannot be made water-soluble by neutralization with alkali or othermeans, but they can be utilized in the form of aqueous dispersions byuse of an emulsifier or other additives in the usual manner.

In order to form an aqueous solution and/or an aqueous dispersion ofsuch resins, varied methods known in the art may be adopted. Forexample, some resins of high acid values can be made soluble in water byneutralization with ammonia or the like. A resin which cannot be madewater-soluble in that way is once dissolved in a solvent or fluidized byheating to a molten state, and then mixed with water, with vigorousstirring in the presence of an emulsifying dispersant, until an aqueousdispersion results. The solvent, where used, is removed subsequently byheating and/or evacuation.

I. Reaction products of a compound having one or more hydroxyl groups inthe molecule, a polyisocyanate compound, and a monomer having a groupreactive with the isocyanate group and at least one radiation-curable,unsaturated double bond, at a molecular ratio of one to one or more toone or more. An example is a resin (prepolymer, oligomer, or teromer)having two acrylic double bonds at the molecular PG,9 ends, prepared byreacting two moles of toluene diisocyanate with one mole of abifunctional polyether (marketed by Asahi Denka Kogyo K.K. under thetrade designation "Adeka Polyether P-1000"), which is made by addingpropylene oxide to propylene glycol, and further reacting the resultantproduct with two moles of 2-hydroxyethyl methacrylate.

The compounds having one or more hydroxyl groups which may be employedinclude polyfunctional polyethers, such as "Adeka Polyether P-700","-P-1000", and "-G-1500" (all made by Asahi Denka Kogyo), and "Polymeg1000" and "-650" (both made by the Quaker Oats Co.); cellulosederivatives, such as nitrocellulose, acetylcellulose, and ethylcellulose; hydroxyl-containing, partly saponified vinyl chloride-vinylacetate copolymers, such as "Vinylite VAGH" (marketed by Union CarbideCorp. of the U.S.); polyvinyl formal; polyvinyl butyral; polyfunctionalpolyethers, such as "Polycaprolactone PCP-0200", "-PCP-0240", and"-PCP-0300" (all made by Union Carbide Corp.); saturated polyesterresins prepared by the ester linkage of a saturated polybasic acid, suchas phthalic, isophthalic, terephthalic, adipic, succinic, or sebacicacid, with a polyhydric alcohol, such as ethylene glycol, diethyleneglycol, 1,4-butanediol, 1,3-butanediol, 1,2-propylene glycol,dipropylene glycol, 1,6-hexane glycol, neopentyl glycol, glycerin,trimethylolpropane, or pentaerythritol; and acrylic polymers containingat least one type of hydroxyl-containing acrylic or methacrylic estersas the polymer component.

Examples of the polyisocyanate compounds useful for the invention are2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,4-xylenediisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, and "Desmodur L"and "-IL" (both made by Farbenfabriken Bayer AG of West Germany).

The compounds having groups reactive with the isocyanate group and alsohaving at least one radiation-curable, unsaturated double bond include:hydroxyl-containing esters, such as 2-hydroxyethyl ester,2-hydroxypropyl ester, and 2-hydroxyoctyl ester, of acrylic andmethacrylic acids; monomers having active hydrogen reactive with theisocyanate group and also having acrylic double bonds, such asacrylamide, methacrylamide, and N-methylolacrylamide; and compoundshaving active hydrogen reactive with the isocyanate group and alsocontaining at least one radiation-curable, unsaturated double bond, suchas allyl alcohols, maleic acid esters of polyhydric alcohols, and mono-and diglycerides of long-chain fatty acids having unsaturated doublebonds.

II. Reaction products of a compound having one or more epoxy groups inthe molecule and a monomer having a group reactive with the epoxy groupand at least one radiation-curable, unsaturated double bond, at amolecular ratio of one to one or more. An example is a resin,prepolymer, or oligomer in which an acrylic double bond is held inpendant fashion in the molecule, prepared by reacting acrylic acid withan epoxy-containing thermoplastic resin obtained by radicalpolymerization of glycidyl methacrylate, and thereby effecting aring-opening reaction of the carboxyl and epoxy groups. Another exampleis a resin (prepolymer or oligomer) prepared by reacting maleic acid andeffecting a ring-opening reaction of the carboxyl and epoxy groups sothat the product has at least one radiation-curable, unsaturated doublebond in the molecular structure.

Examples of the compounds having one or more epoxy groups in themolecule are homopolymers, or copolymers with other polymerizablemonomers, of an epoxy-containing acrylic or methacrylic ester, such asglycidyl acrylate or glycidyl methacrylate; "Epikote 828", "-1001","-1007", and "-1009" (all made by Shell Chemical Co.); and various typesof epoxy resins.

Among the compounds having a group reactive with the epoxy group and atleast one radiation-curable, unsaturated double bond are:carboxyl-containing acrylic monomers, such as acrylic and methacrylicacids; primary- or secondary-amino-containing acrylic monomers, such asmethylaminoethyl acrylate and methylaminomethacrylate; and polybasicacid monomers having at least oen radiation-curable, unsaturated doublebond, such as maleic, fumaric, crotonic, and undecylenic acids.

III. Reaction products of a compound having one or more carboxyl groupsin the molecule and a compound having a group reactive with the carboxylgroup and at least one radiation-curable, unsaturated double bond, at amolecular ratio of one to one or more. An example is a resin (prepolymeror oligomer) prepared by reacting glycidyl methacrylate with acarboxyl-containing thermoplastic resin obtained by solutionpolymerization of methacrylic acid, and effecting a ring-openingreaction of the carboxyl and epoxy groups in the same manner as in II.above, thus introducing an acrylic double bond into the molecule of theproduct.

The compounds containing one or more carboxyl groups in the moleculeare, e.g.: polyesters having a carboxyl group or groups in the molecularchain or at the molecule ends; and homopolymers of radicallypolymerizable and carboxyl-containing monomers, such as acrylic acid,methacrylic acid, maleic anhydride, and fumaric acid, and copolymers ofsuch monomers with other polymerizable monomers.

Examples of the compounds having a group reactive with the carboxylgroup and at least one radiation-curable, unsaturated double bond areglycidyl acrylate and glycidyl methacrylate.

IV. Polyester compounds containing at least one radiation-curable,unsaturated double bond in the molecular chain. An example is anunsaturated polyester resin (prepolymer or oligomer) having at least oneradiation-curable, unsaturated double bond, which consists of asaturated polyester resin formed by ester linkage of a polybasic acidand a polyhydric alcohol as classified in I. above, with part of thepolybasic acid being replaced by maleic acid.

The polybasic acid and polyhydric alcohol components of the saturatedpolyester resin may be any of the compounds mentioned in I. above, andthe radiation curable, unsaturated double bond may be that of maleicacid, fumaric acid, or the like.

The radiation-curable, unsaturated polyester resin is made in the usualmanner from at least one type each of polybasic acid and polyhydricalcohol components with the addition of maleic acid, fumaric acid, orthe like. In the presence of a catalyst the starting material mixture isallowed to react for dehydration or dealcoholation in a nitrogenatmosphere at a temperature in the range of 180°-200° C., and, followingthe conclusion of the reaction, the temperature is raised to 240°-280°C., and then condensation is effected under reduced pressure of 0.5-1.0mmHg to obtain the objective polyester resin. The content of maleic,fumaric, or other acid in the starting mixture is in the range of 1-40mol%, preferably in the range of 10-30 mol%, in the total acid amountfor the facility of cross-linking and for desirable radiation curabilityduring the course of manufacture of the magnetic recording medium.

V. Depending on the intended uses of the products, low-molecular-weightcompounds having at least one radiation-curable, unsaturated double bondmay also be employed. Examples of those low-molecular-weight compoundsare styrene, ethylacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate, diethylene glycol diacrylate, diethylene glycoldimethacrylate, 1,6-hexane glycol diacrylate, 1,6-hexane glycoldimethacrylate, trimethylolpropane triacrylate, trimethylolpropanetrimethacrylate, polyethylene glycol (meth)acrylate, and polypropyleneglycol di(meth)acrylate.

The thermoplastic resin free from any unsaturated double bond curableupon irradiation may be any aqueously dispersible resin of rubber oracrylic type. Examples are styrene-butadiene copolymer, acrylic estercopolymer, vinyl chloride-vinyl acetate copolymer,styrene-butadiene-acrylic acid copolymer, acrylonitrile-butadienecopolymer, polyurethane, polyvinylidene chloride, and vinylidenechloride-acrylic acid copolymer.

Such a resin may be obtained either as an aqueous dispersion as thedirect product or by aqueously dispersing by the use of an emulsifyingdispersant before using the product in the manner already described. Inthe case of a thermoplastic, water-soluble resin, any of natural,synthetic, or semisynthesized resins may be employed. Examples arepolyvinyl alcohol, carboxymethyl cellulose, sodium acrylate copolymer,isobutylene-maleic anhydride ammonium salt copolymer, styrene-maleicanhydride ammonium salt copolymer, and vinylpyrrolidone copolymer.

In practicing the invention, water is utilized as a solvent ordispersion medium. Alternatively, for improved applicability on the baseof polyester film or the like, part of the water may be replaced by asolvent compatible with water, e.g., isobutanol, butanol, ethylcellosolve, diacetone alcohol, 3-methoxybutanol, isopropyl cellosolve,or butyl cellosolve.

The substrate to be coated with the magnetic coating material inaccordance with the present invention may be the polyethyleneterephthalate film that is in wide use as a base material for magneticrecording media. For applications where heat resistance is a necessity,a polyimide film, polyamide film or the like may be utilized. In thecase of a polyester film as a thin base, it is often used aftermonoaxial or biaxial stretching. Other applications include coating ofpaper.

The magnetic powders which may be used in the present invention areγ-Fe₂ O₃, Fe₃ O₄, Co-doped γ-Fe₂ O₃, Co-doped γ-Fe₂ O₃ -Fe₃ O₄ solidsolution, Co-base-compound-coated γ-Fe₂ O₃ - and Co-base-compound-coatedFe₃ O₄ (including those oxidized to an intermediate state between itselfand γ-Fe₂ O₃). (The term "Co-base compound" as used herein means cobaltoxide, cobalt hydroxide, cobalt ferrite, cobalt ion-adsorbates and thelike which enable the magnetic powder to take advantage of the magneticanisotropy of cobalt in improving its coercive force.)

For the radiation-curable, magnetic coating material of the invention,it is advisable to use various additives usually employed for thoseapplications, such as an anti-static agent, lubricant, dispersant,sensitizer, leveling agent, wear-resisting agent, and film-reinforcingagent, to suit the particular end uses.

The radioactive rays for use in cross-linking and curing the magneticcoating film in accordance with the invention may be electron beams froman electron-beam accelerator as the source, γ-rays from Co⁵⁰, β-raysfrom Sr⁹⁰, X-rays from an X-ray generator, or the like. The electronbeams from an electron-beam accelerator are most advantageously usedbecause of the ease of controlling the absorbed dose, introducing thesource into the process line, and providing shield from the ionizingradiation.

With regard to the characteristics of electron beams for use in curingthe magnetic coating film, it is desirable from the standpoint oftransmissivity to adopt an electron-beam accelerator which operates withan acceleration voltage of 100-750 KV, preferably 150-300 KV, and effectthe irradiation so that the absorbed dose may be in the range of 0.5-10megarads.

In the case of a magnetic tape, in particular, the film of coating to becured is thin, and therefore the adoption of the low-dosageelectron-beam accelerator made by Energy Science Co. of the U.S.(trademarked "Electron-curtain system") or the equivalent is extremelyadvantageous because of the ease with which the accelerator can beinstalled on the tape coating line and the shield can be establishedagainst secondary X-rays inside the apparatus.

Of course, the Van de Graff accelerator in extensive use as anelectron-beam accelerator may be employed instead.

In radiation curing, exposure of the magnetic coating film to radiationin a stream of N₂, He, or other inert gas is important. A film ofmagnetic coating, which is very thickly filled with a magnetic pigment,is highly porous in structure. When it is exposed to electron beams inair, O₃ and the like produced by the irradiation for the cross-linkingof the binder components will have an adverse effect, keeping theradicals formed in the polymer from effectively acting in thecross-linking reaction. The adverse effect is exercised not merely onthe surface of the magnetic coating layer but also deep into the porousfilm, thus hampering the cross-linking of the binder. For this reason itis essential to maintain an inert gas atmosphere of N₂, He, CO₂ or thelike around the portion to be irradiated.

Next, the present invention is illustrated by the following examples andcomparative examples. Throughout these examples the "parts" and "%" areall by weight.

Before proceeding to the examples, typical methods of synthesizingresins for use in the invention will be explained below:

Example of resin synthesis (a)

One hundred parts of a copolymer of vinyl chloride, vinyl acetate, andvinyl alcohol at a ratio by weight of 93:2:5, having a molecular weightof 18000 was dissolved with heat in a mixed solvent consisting of 150parts each of toluene and methyl ethyl ketone. After the addition of0.002 part each of tin octylate and hydroquinone to the solution, themixture was heated to 80° C. under a stream of nitrogen, and with theaddition of 7.5 parts of tolylene diisocyanate (TDI) to be describedlater, the mixture was allowed to react at 80° C. for 5 hours tosynthesize a base resin. Next, 6 g of polyvinyl alcohol and 6 g of anonionic surfactant (marketed by Daiichi Kogyo Seiyaku Co. under thetrade designation "Evane 785") were dissolved in 1 kg of water, and theresin solution prepared above was slowly flown, with stirring by ahomomixer, into this aqueous solution. Following the completion of thisaddition, the mixture was agitated for a further period of 30 minutesfor emulsification. The mixed solution was heated and boiled to removethe solvent, and excess water was distilled away to adjust the solidcontent to 30%. Thus, an acrylic double bond-containing vinylchloride-vinyl acetate copolymer emulsion (a) was obtained.

(Synthesis of TDI adduct)

In a one-liter flask 348 parts of tolylene diisocyanate (TDI), 0.07 partof tin octylate, and 0.05 part of hydroquinone were heated to 80° C.under a nitrogen stream. With stirring, 260 parts of 2-hydroxyethylmethacrylate (2HEMA) was added dropwise to the above mixture over 2hours. After the addition, the whole mixture was allowed to react for 3hours, and a 2HEMA adduct of TDI was obtained.

Example of resin synthesis (b)

A four-necked flask was charged with 237 parts of phthalic anhydride,39.2 parts of maleic anhydride, 108 parts of 1,3-butylene glycol, 134parts of dipropylene glycol, and 0.1 part of hydroquinone. In a nitrogenatmosphere at 180° C. the mixture was allowed to react, with stirring,for 8 hours to synthesize a base resin. Next, 50 g of polyvinyl alcoholand 10 g of a nonionic surfactant ("Evane 785") were dissolved in 1 kgof water, and the solution was heated to 80° C., and then, with stirringby a homomixer, the above resin was gradually added to this aqueoussolution. After the addition, the mixture was further agitated for 30minutes, and an unsaturated polyester emulsion (b) with a solid contentwas 30% was obtained.

Example of resin synthesis (c)

One hundred parts of isopropyl alcohol was charged into a four-neckedflask and, with stirring at 80° C., 30 parts of methyl acrylate, 40parts of n-butyl acrylate, 30 parts of methacrylic acid, and 0.5 part oft-butyl perbenzoate were added dropwise over 3 hours. After theaddition, the whole mixture was agitated at 80° C. for 3 hours. Next, 17parts of glycidyl methacrylate, 1 part of tetraethylammonium chloride,and 0.1 part of hydroquinone were added to the mixed solution, and thewhole mixture was allowed to react at 80° C. for 10 hours to obtain abase resin. To the resulting resin were added 37 parts of an aqueoussolution of 29% ammonia and then 224 parts of water, with stirring, foremulsification. An aqueous solution (c) of a water-soluble resin with asolid content of 25% resulted.

Example of resin synthesis (d)

Fifty parts each of toluene and methyl ethyl ketone were charged into afour-necked flask. While the mixture was being agitated at 80° C., 40parts each of methyl methacrylate and n-butyl acrylate, 20 parts of2-hydroxyethyl methacrylate, and 0.5 part of t-butyl perbenzoate wereadded dropwise over 3 hours. After the dropping, the whole mixture washeated at 80° C. and agitated for 3 hours. Next, 22 parts of a TDI-allylalcohol adduct prepared in the same way as in Example of resin synthesis(a) was added and then 0.005 part each of tin octylate and hydroquinonewere added. The whole mixture was allowed to react at 80° C. for 5 hoursto obtain a base resin. Generally in conformity with the procedure ofExample of resin synthesis (a), the base resin was emulsified in 1 kg ofan aqueous solution of the nonionic surfactant and polyvinyl alcohol,the emulsion was boiled to remove the solvent, and the water wasdistilled away, leaving a solid content of 25% in an allyl-containingacrylic emulsion (d) as the product.

Example of resin synthesis (e)

A flask was charged with 250 parts of ε-caprolactone polyol "PCL 220"(trademark of a Daicel Ltd. product with a molecular weight of 2000),122.2 parts of 2-hydroxyethyl methacrylate, 0.024 part of hydroquinone,and 0.033 part of tin octylate. The charge was heated and dissolved at80° C., and 163.6 parts of TDI was gradially dropped into the solution.After the addition, the whole mixture was allowed to react, withstirring, at 80° C. for 5 hours to prepare a base resin. Then, generallyin conformity with the procedure of Example of resin synthesis (b), thebase resin was emulsified with a surfactant and an aqueous polyvinylalcohol solution. An acrylic double bond-containing urethane prepolymeremulsion (e) with a solid content of 30% was obtained.

Example of resin synthesis (f)

A flask was charged with 277 parts of trimethylolpropane, 264 parts ofisophthalic acid, and 87 parts of trimellitic anhydride, and the chargewas allowed to react, with stirring, in a nitrogen atmosphere at 180° C.for 4 hours. The resultant was cooled to 110° C., and was reacted withthe addition of 50 parts of xylene, 0.7 part of hydroquinone, 3 parts ofp-toluenesulfonic acid, and 117 parts of acrylic acid for 15 hours toobtain a base resin. By the addition of 5 parts of an aqueous solutionof 29% ammonia and then of 1500 parts of water to the base resin, anaqueous solution (f) of a water-soluble polyester acrylate with a solidcontent of 30% was obtained.

Example of resin synthesis (g)

A mixture consisting of 200 parts of "Epikote 828" (trademark of anepoxy resin marketed by Shell Chemical Co.), 1 part ofN,N-dimethylbenzylamine, and 0.5 part of hydroquinone was heated to 100°C. and, with stirring, 72 parts of acrylic acid was added dropwise over2 hours. After the addition, the mixture was allowed to react for 15hours to an acid value of not more than 5. Then, 98 parts of maleicanhydride was added and a reaction was effected at 100° C. for 5 hoursto obtain a base resin. With the addition of 70 parts of an aqueoussolution of 29% ammonia and then 1050 parts of water, an aqueoussolution (g) of an acrylic double bond-containing, water-soluble epoxyprepolymer with a solid content of 25% was obtained.

EXAMPLE 1

A composition consisting of

    ______________________________________                                        cobalt-coated acicular γ-Fe.sub.2 O.sub.3                                                        120    parts                                         (length 0.4 μm, breadth 0.05 μm,                                        Hc 600 Oe)                                                                    carbon black (for antistatic use,                                                                      5      parts                                         ("Mitsubishi Carbon Black MA-600")                                            α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (lecithin derived                                                                     3      parts                                         from purified soybean oil)                                                    water                    100    parts                                         ______________________________________                                    

was mixed up by a ball mill for 3 hours, and the acicular magnetic ironoxide was thoroughly wetted with the dispersing agent.

Next, a binder mixture consisting of

    ______________________________________                                        emulsion (a)            15 parts                                                                     (on solids basis)                                      emulsion (e)            15 parts                                                                     (on solids basis)                                      water                  200 parts                                              lubricant (polyether-modified silicone                                                                3 parts                                               oil marketed by Shin-etsu Chemical Co.                                        under the trade designation "KF-351")                                         ______________________________________                                    

was mixed and dissolved. It was placed into the ball mill where themagnetic powder had been treated in advance and the two were again mixedand dispersed into each other for 42 hours.

The magnetic coating material thus prepared was applied on a 15 μm-thickpolyester film. After magnetic field orientation on a (1600-gauss)permanent magnet, the coating was dried to remove water as the solventby an infrared lamp or hot air, smoothed on the surface, and washardened by irradiation with electron rays in an N₂ atmosphere by an ESI"Electro-curtain" type electron-beam accelerator at an accelerationvoltage of 150 KeV with an electrode current of 15 mA to a total dosageof 5 mrads.

The resulting film was slit into a 1/2 in.-wide ribbon as a video tape(Specimen #1).

COMPARATIVE EXAMPLE 1

A composition consisting of

    ______________________________________                                        cobalt-coated acicular γ-Fe.sub.2 O.sub.3                                                        120    parts                                         (length 0.4 μm, breadth 0.05 μm,                                        Hc 600 Oe)                                                                    carbon black (for antistatic use,                                                                      5      parts                                         ("Mitsubishi Carbon Black MA-600")                                            α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (lecithin derived                                                                     3      parts                                         from purified soybean oil)                                                    water                    100    parts                                         ______________________________________                                    

was mixed in a ball mill for 3 hours, and the acicular magnetic ironoxide was thoroughly wetted with the dispering agent.

Next, a mixture consisting of

    ______________________________________                                        Nisshin Chemical Ind.'s vinyl chloride-                                                               25 parts                                              vinyl acetate copolymer emulsion                                                                     (on solids basis)                                      (trademark "Vinyplan 200A")                                                   water                  200 parts                                              lubricant ("KF-351")    3 parts                                               ______________________________________                                    

was mixed and dissolved. It was placed into the ball mill where themagnetic powder had been treated in advance and the two were again mixedand dispersed into each other for 42 hours.

After the dispersion of the magnetic coating material, 5 parts (in termsof the solids) of an aqueous dispersion* of an isocyanate compound(marketed by Nippon Polyurethane Ind. Co. under the trade designation"Coronate AP Stable") which is cross-linkable by a reaction with thefunctional groups based on hydroxyl groups of the binder in the coatingcomposition was added to the material and mixed for 20 minutes in theball mill.

The magnetic coating material thus prepared was applied on a 15 μm-thickpolyester film. After magnetic field orientation on a (1600-gauss)permanent magnet, the coating was dried to remove water as the solventby an infrared lamp or hot air, smoothed on the surface, and the filmroll was held in an open kept at 100° C. for 48 hours to promote theisocyanate-induced cross-linking reaction.

The tape so obtained was slit into a 1/2 in.-wide tape as a video tape(Specimen A).

*Preparation of an aqueous solution of Coronate AP Stable

Fifty parts of Coronate AP Stable was dissolved in 100 parts ofcellosolve acetate. Into this solution, with stirring, a solution of 2 gof a nonionic surfactant ("Evane 785") in 200 g of water was introduceddropwise, so that an emulsion of a crosslinking agent with a solidcontent of 14% was obtained.

Table 1 shows the magnetic properties and electromagnetic conversioncharacteristics of the video tapes thus made.

                  TABLE 1                                                         ______________________________________                                        Video tape characteristics                                                                                     Electromag.                                             Br/           Sfce..sup.(2)                                                                         convn. char..sup.(3)                         Hc.sup.(1)                                                                              Br.sup.(1)                                                                           Bm.sup.(1)                                                                           Brl/Br.sup.(1)                                                                       propty.                                                                             4 MHz Y-S/N                              (Oe)      (gauss)        (dB)    (dB)  (dB)                                   ______________________________________                                        Example                                                                              660    1600   0.83 2.10   +6.5  +1.5  +2.0                             (Spec.                                                                        #1)                                                                           Comp.  670    1450   0.81 2.00   +5.0  0     0                                Ex.                                                                           (Spec. A)                                                                     ______________________________________                                         Notes:                                                                        .sup.(1) Measured by Toei Kogyo's orientation meter, Model "VSMIII", with     the application of a magnetic field of 5000 G.                                .sup.(2) The degree of surface gloss based on that of a reference glass       plate which was 0 dB.                                                         .sup.(3) Determined with the test tape set on a Matsushita video deck,        Model "National Maclord 6600", on the basis of the data of Comparative        Example (Spec. A) as 0 dB.                                               

As is clear from the values of squareness ratio (Br/Bm), orientation(Br'/Br"), and residual flux density (Br) in Table 1, the electronbeam-cured binder was equivalent to or superior to the thermally curedbinder in dispersibility for the magnetic particles. Also, with theelectron beam curing there occurred no transfer of the surfacecoarseness of the back surface of the base to the magnetic coatingsurface as observed in the course of thermal curing. Thus, because themagnetic coating surface maintains good surface quality, the electronbeam-cured tape (Specimen #1) showed improvements over the thermallycurved tape (Specimen A) in electromagnetic conversion characteristics,such as the RF sensitivity at 4 MHz and the Y-S/N.

FIG. 1 is a graph showing the rates of attenuation (on stillreproduction) of signals with respect to the reproduction output, asmeasured by recording signals on the video tape specimens by a videotape recorder (Matsushita Electric's Model "NV-3120") for open reelsconforming to the EIAJ unified standards and then by reproducing thestill images with a tension of 200 grams applied on the takeup side by aspring balancer.

As is obvious from the graph, the cross-linking of the binder byirradiation with electron beams gave a tough coating surface with aremarkably reduced signal attenuation, which withstood severe abrasiveconditions such that the relative velocity of the magnetic coatingsurface and the head was as high as 11 m/sec.

FIG. 2 is a graphic representation of changes in friction of test videotapes with running time. Each tape was held for 5 days under changingconditions in 5 cycles, each starting with a temperature of -10° C. anda relative humidity of 0% and ending with 60° C. and 80%. After thisperiod, the specimen was allowed to stand at room temperature for 24hours. It was then set on the same video tape recorder as used for thestill reproduction test, a tension analyzer marketed by Nippon JidoSeigyo Co., Model "IVA-500", was set between the head drum and the pinchroller of the recorder, and the changes in tension on the takeup side ofthe test tape with running time were measured.

This test permits evaluation of not only the friction coefficient levelof the magnetic film itself of each test tape but also the degree ofdeterioration of tape running quality with exudation of thelow-molecular-weight components from the magnetic film, and stability ofthe tape against the environmental conditions such as temperature andhumidity.

As can be seen from the figure, the electron beam-cured tape of Specimen#1 showed a low friction coefficient and adequate cross-linking with thelow-molecular-weight elastomer component. With no exudation to thecoating surface, the tape possessed good running stability againstchanges in temperature and humidity.

As compared with Specimen #1, Specimen A was somewhat unstable,supporting the uniformity of the electron beam-induced cross-linking.

EXAMPLE 2

A composition consisting of

    ______________________________________                                        Fe-alloy acicular magnetic powder                                                                      120    parts                                         (length 0.3 μm, breadth 0.04 μm,                                        Hc 1100 Oe)                                                                   carbon black             5      parts                                         ("Mitsubishi Carbon Black MA-600")                                            α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (sodium oleate)                                                                       2      parts                                         water                    100    parts                                         ______________________________________                                    

was mixed for 3 hours, and the fine magnetic alloy powder was thoroughlywetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion ("Vinyplan 200A")                                                                           15 parts                                                                     (on solids basis)                                       aqueous solution (f)   15 parts                                                                     (on solids basis)                                       water                 200 parts                                               lubricant (Shin-etsu Chemical's polyester-                                                           3 parts                                                modified silicone oil trade designation                                       "KF-351")                                                                     ______________________________________                                    

was mixed with the previously treated magnetic powder composition by ahigh speed mixer for 70 minutes, and the resulting mixture was dispersedin a sand mill for 4 hours.

The magnetic coating material thus prepared was applied on a 15 μm-thickpolyester film. After magnetic field orientation, solvent removal bydrying, and surface smoothing, the coating was hardened by irradiationwith electron beams in an N₂ atmosphere by an ESI "Electro-curtain" typeelectron-beam accelerator at an acceleration voltage of 150 KeV with anelectrode current of 15 mA to a total dosage of 5 mrads.

The resulting film was slit into a 1/2 in.-wide ribbon as a video tape,Specimen #2.

COMPARATIVE EXAMPLE 2

A composition consisting of

    ______________________________________                                        Fe--alloy acicular magnetic powder                                                                     120    parts                                         carbon black             5      parts                                         ("Mitsubishi Carbon Black MA-600").                                           α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (sodium oleate)                                                                       2      parts                                         water                    100    parts                                         ______________________________________                                    

was mixed up for 3 hours, and the fine magnetic alloy powder wasthoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion ("Vinyplan 200A")                                                                        25 parts                                                                     (on solids basis)                                          water              200 parts                                                  lubricant           3 parts                                                   ______________________________________                                    

was mixed with the previously treated magnetic powder composition by ahigh speed mixer for 70 minutes, and the resulting mixture was dispersedin a sand mill for 4 hours. After the dispersion the magnetic coatingmaterial was mixed with 5 parts by weight (in terms of solids) of anaqueous dispersion of Nippon Polyurethane's "Coronate AP Stable" for 20minutes.

The magnetic coating material so obtained was applied on a 15 μm-thickpolyester film. After magnetic field orientation, solvent removal bydrying, and surface smoothing the film roll was held in an oven kept at100° C. for 48 hours to promote the cross-linking reaction with the aidof the isocyanate.

The film obtained was slit into a 1/2 in.-wide ribbon as a video tape,Specimen B.

Table 2 shows the properties of the alloy video tapes.

                  TABLE 2                                                         ______________________________________                                        Properties of alloy video tapes                                                           Electromag.  Recipro. run                                                                             No. of                                    Surface     convn. charac..sup.(2)                                                                     durability.sup.(3)                                                                       dropout.sup.(4)                           quality     4 MHz    Y-S/N   at 40° C. 60%                                                                   per min.                                ______________________________________                                        Example                                                                              13.0     +2.0     +3.0  over 250 runs                                                                          0.5                                   (Spec.                                                                        #2)                                                                           Comp.  10.0     0        0     200 runs 1.2                                   Ex.                                                                           (Spec. B)                                                                     ______________________________________                                         Notes:                                                                        .sup.(1) The degree of surface gloss based on that of a reference glass       plate which was 0 dB.                                                         .sup.(2) Determined with the test tape set on a video deck, "National         Baclord 6600", on the basis of the data of Comparative Example, Spec. B,      as 0 dB.                                                                      .sup.(3) Ditto                                                                .sup.(4) Ditto                                                           

As Table 2 clearly indicates, the tape Specimen #2 was far superior toSpecimen B in electromagnetic conversion characteristics. This meansthat the electron-beam curing eliminated the transfer of the surfacecoarseness of the back side of the base to the magnetic coating surface,a phenomenon that occurs in the course of thermal curing.

Also, while Specimen #2 used as one of the binder components a resinhaving no radiation-curable, unsaturated double bond, the coatingmaterial was so thoroughly cross-linked that the tape exhibitedexcellent properties such as reciprocating running durability under hotand humid conditions and the fewer frequency of dropouts.

EXAMPLE 3

A composition consisting of

    ______________________________________                                        Fe-alloy acicular magnetic powder                                                                     120    parts                                          (length 0.3 μm, breadth 0.04 μm,                                        Hc 1100 Oe)                                                                   dispersing agent (sodium oleate)                                                                      2      parts                                          water                   100    parts                                          ______________________________________                                    

was mixed well with a powerful mixer for 3 hours, and the fine magneticalloy powder was thoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion (b)              18 parts                                                                     (on solids basis)                                    emulsion (e)              12 parts                                                                     (on solids basis)                                    water                    200 parts                                            lubricant (polyethylene glycol monostearate                                                             3 parts                                             marketed by Nikko Chemicals Co. under the                                     trade designation "NIKKOL MYS-40).                                            ______________________________________                                    

was thoroughly mixed and dissolved.

The resultant was mixed with the previously treated magnetic powdercomposition by a high speed mixer for 70 minutes, and the mixedcomposition was mixed and dispersed by a sand mill for 4 hours.

The magnetic coating material thus prepared was applied on a 12 μm-thickpolyester film. After magnetic field orientation, solvent removal bydrying, and surface smoothing, the coating was hardened by irradiationwith electron beams in an N₂ atmosphere by an "Electro-curtain" typeelectron-beam accelerator at an acceleration voltage of 150 KeV with anelectrode current of 15 mA to a total dosage of 5 mrads.

The resulting film was slit into a 3.8 mm-wide ribbon as an alloy-coatedaudio cassette tape (Specimen #3).

EXAMPLE 4

A composition consisting of

    ______________________________________                                        Fe-alloy acicular magnetic powder                                                                     120    parts                                          (length 0.3 μm, breadth 0.04 μm,                                        Hc 1100 Oe)                                                                   dispersing agent (sodium oleate)                                                                      2      parts                                          water                   100    parts                                          ______________________________________                                    

was mixed well with a powerful mixer for 3 hours, and the fine magneticalloy powder was thoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        Seiko Chemical's acrylic emulsion                                                                   10 parts                                                (trademark "Hilos X. T-43")                                                                        (on solids basis)                                        aqueous solution (c)  10 parts                                                                     (on solids basis)                                        aqueous solution (g)  10 parts                                                                     (on solids basis)                                        water                200 parts                                                lubricant ("NIKKOL MYS-40")                                                                         3 parts                                                 ______________________________________                                    

was thoroughly mixed and dissolved.

The resultant composition was mixed with the previously treated magneticpowder composition by a high speed mixer for 70 minutes, and was furthermixed and dispersed in a sand mill for 4 hours.

The magnetic coating material prepared in this way was applied on a baseto form a tape in the same manner as described in Example 3.

The tape was then slit into a ribbon 3.8 mm in width, and analloy-coated audio cassette tape (Specimen #4) was obtained.

EXAMPLE 5

A composition consisting of

    ______________________________________                                        Fe-alloy acicular magnetic powder                                                                     120    parts                                          (length 0.3 μm, breadth 0.04 μm,                                        Hc 1100 Oe)                                                                   dispersing agent (sodium oleate)                                                                      2      parts                                          water                   100    parts                                          ______________________________________                                    

was mixed well with a powerful mixer for 3 hours, and the fine magneticalloy powder was thoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion (a)         30 parts                                                                     (on solids basis)                                         water               200 parts                                                 lubricant ("NIKKOL MYS-40")                                                                        3 parts                                                  ______________________________________                                    

was thoroughly mixed and dissolved. The resultant composition was mixedwith the previously treated magnetic powder composition by a high speedmixer for 70 minutes, and was further mixed and dispersed in a sand millfor 4 hours.

The magnetic coating material thus obtained was applied on a base tomake a tape in the same manner as in Example 3.

The tape was slit into a 3.8 mm-wide ribbon as an alloy-coated audiocassette tape (Specimen #5).

Table 3 summarizes the characteristics of the alloy-coated audiocassette tapes made in the foregoing way. Specimen #3 that used a binderblending an electron beam-sensitive component with a molecular weight of5000 or more with an electron beam-sensitive component with a molecularweight of less than 5000, and Specimen #4 that used a binder blendingthe above two components with a resin insensitive to the electron beamshad well-balanced characteristics as audio tapes and exhibited goodbinder affinity for the magnetic alloy powder, superior surfaceformability of the coating, and very smooth surface with high residualflux density, as compared with Specimen #5 that used a binder comprisingonly the electron beam-sensitive resin with a molecular weight of notless than 5000.

Thus, tapes of very high densities over MOL ranges from the lowfrequency of 333 Hz to the high frequency of 16 KHz were obtained.

Further, in the physical properties that represent the reliability oftapes, i.e., the running time up to the point where the tape begins tosqueal and the durability in repeated, reciprocating passages on a carstereo set, the tapes that used the mixed binder of resins of differentmolecular weights properly combined rigidity and softness due to goodcurability, and hence they proved superior in those properties to thetape, Specimen #5, that used only a resin having a molecular weight ofover 5000 as the binder.

                                      TABLE 3                                     __________________________________________________________________________    Properties of audio cassette tapes                                                                Electromag.                                                                   convn. chrac..sup.(3)                                                                      Tape                                                                              Recipro.                                                 Surface                                                                           MOL MOL MOL  squeal                                                                            run dura-                                                qlty..sup.(2)                                                                     333 Hz                                                                            8 KHz                                                                             16 KHz                                                                             start.sup.(4)                                                                     bility.sup.(5)                           Ex.                                                                              Hc.sup.(1) (Oe)                                                                     Br.sup.(1) (gauss)                                                                   (dB)                                                                              (dB)                                                                              (dB)                                                                              (dB) (hr)                                                                              (No. of run)                             __________________________________________________________________________    #3 1060  3400   212.5                                                                             +3.2 +6.0                                                                             +11.0                                                                              24  Stopped aft.                                                                  230 runs.                                #4 1060  3200   12.0                                                                              +3.0 +5.5                                                                             +10.5                                                                              24  Over 250                                                                      runs.                                    #5 1070  3000   11.0                                                                              +2.7 +4.7                                                                              +9.5                                                                              20  Stopped aft.                                                                  180 runs.                                __________________________________________________________________________     Notes:                                                                        .sup.(1) Measured by Toei Kogyo's orientation meter, Model "VSMIII" with      the application of a magnetic field of 5000 G.                                .sup.(2) The degree of surface gloss based on that of a reference glass       plate which was 0 dB.                                                         .sup.(3) Measured with the test tape set in the metal position on             Nakamichi Kenkyusho's cassette deck, Model "Nakamichi 582".                   .sup.(4) Measured by a car stereo set, Model "National CX318" at              40° C. and 80% RH.                                                     .sup.(5) Measured by Matsushita's car stereo set, Model "CX1147D" at          40° C. and 60% RH.                                                

EXAMPLE 6

A mixture consisting of

    ______________________________________                                        Fe.sub.2 O.sub.3 (length 0.8 μm, breadth 0.2 μm,                                                 120    parts                                         HC 300 Oe)                                                                    carbon black (for antistatic use,                                                                      5      parts                                         "Mitsubishi Carbon Black MA-600").                                            α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (Toho Chemical's                                                                      3      parts                                         polyoxyethylene sorbitan mono-                                                oleate, trademarked "T-80")                                                   water                    100    parts                                         ______________________________________                                    

was mixed up in a ball mill for 3 hours, and the magnetic iron oxideparticles were thoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion (d)           20 parts                                                                     (on solids basis)                                       Shin-Nakamura Chemical's "NK Ester                                                                   10 parts                                               A-14G" (acrylic monomer)                                                      water                 200 parts                                               lubricant ("KF-351")   3 parts                                                ______________________________________                                    

was thoroughly mixed and dissolved.

It was placed into the ball mill where the magnetic powder had beentreated in advance and the two were again mixed and dispersed into eachother for 42 hours.

The magnetic coating material thus obtained was applied on one side of a188 μm-thick polyester film to form a coat about 10 μm thick. Thecoating layer was dried and smoothed on the surface, and then hardenedby irradiation with electron beams in an N₂ atmosphere by an"Electron-curtain" type electron-beam accelerator at an accelerationvoltage of 175 KeV with an electrode current of 15 mA to a total dosageof 2 mrads.

From the roll thus obtained a disc (about 65 mm in diameter) was blankedout as a magnetic disc, Specimen #6.

EXAMPLE 7

A composition consisting of

    ______________________________________                                        Fe.sub.2 O.sub.3 (length 0.8 μm, breadth 0.2 μm,                                                 120    parts                                         Hc 300 Oe)                                                                    carbon black (for antistatic use,                                                                      5      parts                                         "Mitsubishi Carbon Black MA-600").                                            α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (Toho Chemical's                                                                      3      parts                                         polyoxyethylene sorbitan mono-                                                oleate "T-80")                                                                water                    200    parts                                         ______________________________________                                    

was mixed in a ball mill for 3 hours, and the magnetic iron oxide wasthoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        "NK Ester A-14G"        30     parts                                          water                   200    parts                                          lubricant (Shin-etsu Chemical's                                                                       3      parts                                          polyether-modified silicone oil                                               "KF-351")                                                                     ______________________________________                                    

was mixed well and dissolved.

The solution was placed into the ball mill where the magnetic powdercomposition had been treated, and the both were mixed and dispersed for42 hours.

The magnetic coating material thus prepared was applied, dried, smoothedon the surface, and then exposed to electron-beam radiation in the samemanner as in Example 5.

From the resulting roll a disc (about 65 mm in diameter) was blanked outas a magnetic disc, Specimen #7.

Each of the magnetic discs thus obtained was set on arecorder-reproducer and was driven to pass in sliding contact with themagnetic head (at a pad pressure of 40 g/cm²) at a speed of about 1m/sec, and the running time required for the dropouts to reach acumulative number of 1000 was measured.

The results and the surface conditions of the magnetic coating layerswere as shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Wear resistances of magnetic discs                                                         Running time                                                                           Surface condition                                       ______________________________________                                        Ex. (Specimen #6)                                                                            Over 50 hrs.                                                                             No abnormality                                      Ex. (Specimen #7)                                                                            40 hrs.    Some scratches                                      ______________________________________                                    

Specimen #6 that used a binder blending an electron beam-curable resinbaving a molecular weight of not less than 5000 with an electronbeam-curable resin having a molecular weight in the range of 200-5000had proper rigidity and softness, exhibiting very good wear resistanceas a magnetic disc.

On the other hand, Specimen #7 that used the binder comprising only thelow-molecular weight "NK Ester A-14G" had a shorter running time anddeveloped some scratches. This was because the coating film hadsufficient rigidity but was not properly soft and was brittle.

EXAMPLE 8

A composition consisting of

    ______________________________________                                        Fe.sub.2 O.sub.3 (length 0.8 μm, breadth 0.2 μm,                                                 120    parts                                         Hc 300 Oe)                                                                    carbon black (for antistatic use,                                                                      5      parts                                         "Mitsubishi Carbon Black MA-600")                                             α-Al.sub.2 O.sub.3 powder                                                                        2      parts                                         (0.5 μm in particle size)                                                  dispersing agent (Toho Chemical's                                                                      3      parts                                         polyoxyethylene sorbitan mono-                                                oleate "T-80")                                                                water                    200    parts                                         ______________________________________                                    

wss mixed in a ball mill for 3 hours, and the magnetic iron oxide wasthoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        emulsion (b)           15 parts                                                                     (on solids basis)                                       aqueous solution (g)   15 parts                                                                     (on solids basis)                                       water                 200 parts                                               lubricant (Nikko Chemicals' polyethylene                                                             3 parts                                                glycol monostearate "NIKKOL MYS-40")                                          ______________________________________                                    

was thoroughly mixed and dissolved.

The solution was placed in the ball mill where the magnetic powdercomposition had been treated, and the both were mixed and dispersed for42 hours.

The magnetic coating material so prepared was applied on one side of a75 μm-thick polyester film to form a coat about 10 μm thick. The coatinglayer was dried, smoothed on the surface, and was hardened byirradiation with electron beams in a CO₂ gas atmosphere by an"Electron-curtain" type electron-beam accelerator at an accelerationvoltage of 175 KeV with an electrode current of 15 mA to a total dosageof 2 mrads.

The roll thus obtained was cut down to a predetermined size as asealless commutation ticket, Specimen #8.

COMPARATIVE EXAMPLE 3

A composition consisting of

    ______________________________________                                        Fe.sub.2 O.sub.3 (length 0.8 μm, breadth 0.2 μm,                                                  120    parts                                        Hc 300 Oe)                                                                    carbon balck (for antistatic use,                                                                       5      parts                                        "Mitsubishi Carbon Black MA-600")                                             α-Al.sub.2 O.sub.3 powder                                                                         2      parts                                        (0.5 μm in particle size)                                                  dispersing agent (Toho Chemical's "T-80",                                                               3      parts                                        polyoxyethylene sorbitan                                                      monooleate)                                                                   water                     100    parts                                        ______________________________________                                    

was mixed in a ball mill for 3 hours, and the magnetic iron oxide wasthoroughly wetted with the dispersing agent.

Next, a mixture consisting of

    ______________________________________                                        Nippon Zeon Co.'s styrene-butadiene                                                                  30 parts                                               copolymer emulsion "Hycar LX204")                                                                   (on solids basis)                                       water                 200 parts                                               lubricant ("NIKKOL MYS-40")                                                                          3 parts                                                ______________________________________                                    

was thoroughly mixed and dissolved.

This solution was placed in the ball mill where the magnetic powdercomposition had been treated, and the both were mixed and dispersed for42 hours.

The magnetic coating material thus obtained was applied on a 75 μm-thickpolyester film to form a coat about 10 μm thick, and the coating layerwas dried and smoothed on the surface.

The resulting roll was cut down to a predetermined size as a seallesscommunication ticket, Specimen C.

The sealless communication tickets made in the foregoing way were testedfor gate passage endurance, and the results were as given in FIG. 3.

The graph shows the attenuation ratio (Em) of the peak voltage of thereproduction output after each given number of repeated passages to theinitial reproduction output peak voltage which was 100%, and usuallywhen the reproduction peak voltage has decreased below 25% of theinitial reference level, the sealless commutation ticket is regarded tohave closed its service life.

As plotted in FIG. 3, the voltage levels of pulse signal peaks up to30,000 passes of repeated running were measured.

With Specimen C that used a binder based on a thermoplastic resin, thedrop of the output with repeated passage was so serious that the ticketwas of no practical use. In contrast with it, Specimen #8 that used anelectron beam-curable resin manifested remarkable improvement, with amarkedly smaller decline in output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 represent characteristic curves of Examples of thisinvention and of Comparative Examples.

We claim:
 1. A magnetic recording medium which comprises a base and amagnetic coating material consisting essentially of (1) a magneticpowder (2) a first radiation cured resin or mixture of resins which inthe uncured state is a water soluble resin, a water dispersible resin ormixtures thereof having a molecular weight of at least 5000, each ofsaid resins in the uncured state being selected from the groupconsisting of:(a) Reaction products of a compound having at least onehydroxyl group in the molecule, a polyisocyanate compound, and acompound having a group reactive with the isocyanate group and at leastone radiation curable, unsaturated double bond, at a molecular ratio ofone to at least one; (b) Reaction products of a compound having at leastone epoxy group in the molecule and a compound having a group reactivewith the epoxy group and at least one radiation-curable, unsaturateddouble bond, at a molecular ratio of one to at least one; (c) Reactionproducts of a compound having at least one carboxyl group in themolecule and a compound having a group reactive with the carboxyl groupand at least one radiation-curable, unsaturated double bond, at amolecular ratio of one to at least one; and (d) Polyester compoundscontaining at least one radiation-curable, unsaturated double bond,and(3) a second radiation cured resin or mixture of resins which in theuncured state is a water soluble resin, a water dispersible resin ormixtures thereof and is further characterized by the presence of atleast one unsaturated double bond curable upon exposure to radiation andas having a molecular weight of from 200 to
 5000. 2. A magneticrecording medium of claim 1 wherein the second radiation cured resin hasa molecular weight in the uncured state of from 400 to
 3000. 3. Amagnetic recording medium of claim 1 wherein the first radiation curedresin has a molecular weight in the uncured state of not less than 8000.4. A magnetic recording medium of claim 1 wherein the first radiationcured resin has a molecular weight in the uncured state of not less than8000 and the second radiation cured, water soluble resin has a molecularweight in the uncured state of from 400 to 3000.